This invention relates to a method of manufacturing a ferromagnetic metal powder, and more specifically to a method of manufacturing such powder for applications as magnetic recording media.
Well-known ferromagnetic powders that magnetic recording media have hitherto used include maghemite (.gamma.-Fe.sub.2 O.sub.3), cobalt-doped maghemite, magnetite (Fe.sub.3 O.sub.4), cobalt-doped magnetite, iron oxide in the form of an intermediate of magnetite and maghemite, iron oxide as an intermediate of cobalt-doped maghemite and magnetite, and chromium dioxide.
Quality requirements for such media have recently become increasingly stringent, and development of ferromagnetic powders having properties adapted for recording with higher sensitivity and density is now well under way. One of the materials to which the developmental efforts are directed is the ferromagnetic metal powder. With high residual magnetism, the ferromagnetic metal powder promises wide use for high density recording media. A disadvantage is its susceptibility to oxidation due to a large overall surface area of the fine particles. The present invention provides a ferromagnetic powder imparted with excellent magnetic properties for magnetic recording media by subjection of a ferromagnetic metal powder formed by a wet reduction process to a continuous treatment for avoiding the oxidation and improving the properties of the powder.
Conventionally, ferromagnetic metal and alloy powders are made in the following ways:
(1) Thermal decomposition of an organic acid salt of a ferromagnetic metal and reduction of the resultant metal compound with a reducing gas (e.g., Japanese Pat. App. Pub. Nos. 11412/61, 22230/61, and 29280/73). PA1 (2) Reduction of an acicular oxyhydroxide with or without other metal contents, or of an acicular iron oxide obtained from such an oxyhydroxide (e.g., Japanese Pat. App. Pub. Nos. 3862/60 and 1152/62 and Japanese Pat. App. Pub. Discl. No. 82395/73). PA1 (3) Evaporation of a ferromagnetic metal in an inert gas at a low pressure (e.g., Japanese Pat. App. Pub. Nos. 25620/71 and 4131/72, and Japanese Pat. App. Pub. Discl. Nos. 3116/73 and 81092/73). PA1 (4) Pyrolysis of a metal carbonyl compound (e.g., Japanese Pat. App. Pub, Nos. 1004/64, 3415/65, and 16868/70). PA1 (5) Electrodeposition of a ferromagnetic metal powder by means of a mercury cathode and subsequent separation of the product from mercury (e.g., Japanese Pat. App. Pub. Nos. 12910/60, 3860/61, and 19661/70). PA1 (6) Reduction of a metal salt having ferromagnetism by the addition of a reducing agent to a solution of the salt (e.g., Japanese Pat. App. Pub. Nos. 20520/63 and 26555/63 and Japanese Pat. App. Pub. Discl. No. 82396/73). PA1 (1) A hydrated ferromagnetic metal powder is washed with a solvent, such as acetone, so that the water content is replaced by the solvent. This method is disadvantageous because it requires a large quantity of the solvent and yet the water content cannot be thoroughly replaced by the solvent. PA1 (2) A slurry formed by adding acetone to a cake of dehydrated ferromagnetic metal powder is introduced into a container, and the container is placed in a vacuum oven and kept heated at about 150.degree. C. under reduced pressure for tens of hours (Japanese Pat. App. Pub. Discl. No. 41899/74). A problem of the method is that much time is required for the removal of water. In addition, acetone must be used. PA1 (3) A water-containing cake of the ferromagnetic metal powder prepared by wet reduction is washed with an organic solvent, such as acetone, which is miscible with water, and then the cake is gently dried in air for the removal of water (U.S. Pat. Nos. 3,206,338 and 3,535,104, etc.) This method poses a very high possibility of fire where a large volume of the ferromagnetic metal powder is handled. The danger arises from the fact that the metal powder having a large overall surface area and which is highly reactive itself is exposed to air. PA1 (4) A slurry of ferromagnetic metal powder formed by wet reduction is dehydrated, flaked, and fed to a dryer having a heating surface, in which the flakes are dried in an inert atmosphere by the heating surface kept at a temperature between 80.degree. and 250.degree. C., with agitation given for a period at least one third of the drying time (Japanese Pat. App. Pub. Discl. No. 41154/77). The method presents a problem of low productivity because it is essentially a batch treatment process. Moreover, the dehydration and flaking treatments involved necessitate a number of process steps and therefore an increased initial investment in equipment.
The present invention is concerned with a method of manufacturing a composition containing a magnetic metal powder suited for magnetic recording media from the magnetic metal powder obtained by the wet reduction process (6) in particular. Methods of this character, dependent on wet reduction for the supply of the starting material, have had a great difficulty in common. The wet reduction affords a product with a large water content, and it is very important to remove the water from the product in an easy and economical way without impairing the magnetic properties of the resulting powder. None of the prior art methods have, however, proved satisfactory in this respect. For the removal of water the following methods have heretofore been proposed: